Method for treating a mixture in a single-shaft or multishaft mixer corresponding to the preamble of claim 1 and an apparatus therefor.
Methods of this kind are implemented in particular in what are called kneader-mixers. These mixers serve a great diversity of purposes. A first that may be mentioned is that of evaporation of solvent recovery, which takes place batchwise or continuously and often also under reduced pressure. By this means, for example, distillation residues and, in particular, toluene diisocyanates are treated, but also production residues with toxic or high-boiling solvents from chemistry and from drug production, detergent solutions and paint slurries, polymer solutions, elastomer solutions from solution polymerization, adhesives, and sealants.
With the equipment, furthermore, continuous or batchwise contact drying, of water-moist and/or solvent-moist products, is carried out, often likewise under reduced pressure. The application is intended in particular for pigments, dyes, fine chemicals, additives, such as salts, oxides, hydroxides, antioxidants, temperature-sensitive pharmaceutical and vitamin products, active ingredients, polymers, synthetic rubbers, polymer suspensions, latexes, hydrogels, waxes, pesticides, and residues from chemical or pharmaceutical production, such as salts, catalysts, slags, and waste liquors. These methods also find application in food production, as for example in the production and/or treatment of sweetened condensed milk, sugar replacers, starch derivatives, alginates, for the treatment of industrial sludges, oil sludges, biosludges, paper sludges, paint sludges, and generally for the treatment of sticky, crust-forming products of paste-like viscosity, waste products, and cellulose derivatives.
In kneader-mixers, degassing and/or devolatilizing may take place. This operation is applied to polymer melts, after condensation of polyester or polyamide melts, to spinning solutions for synthetic fibers, and to polymer or elastomer pellets or powders in the solid state.
In a kneader-mixer, a polycondensation reaction can take place, usually continuously and usually in the melt, and is used in particular in the treatment of polyamides, polyesters, polyacetates, polyimides, thermoplastics, elastomers, silicones, urea resins, phenolic resins, detergents, and fertilizers.
An addition polymerization reaction may also take place, usually likewise continuously. This is applied to polyacrylates, hydrogels, polyols, thermoplastic polymers, elastomers, syndiotactic polystyrene, and polyacrylamides.
Very generally, solid, liquid, and multiphase reactions can take place in the kneader-mixer. This is especially true of baking reactions, in the treatment of hydrofluoric acid, stearates, cyanates, polyphosphates, cyanuric acids, cellulose derivatives, cellulose esters, cellulose ethers, polyacetal resins, sulfanilic acids, Cu phthalocyanines, starch derivatives, ammonium polyphosphates, sulfonates, pesticides, and fertilizers.
In addition, reactions can take place in solid/gaseous states (e.g., carboxylation) or in liquid/gaseous states. This is applied in the treatment of acetates, acids, Kolbe-Schmitt reactions, e.g., BON, Na salicylates, parahydroxybenzoates, and pharmaceutical products.
Liquid/liquid reactions take place in the case of neutralization reactions and transesterification reactions.
Dissolving and/or degassing in kneader-mixers of this kind takes place in the case of spinning solutions for synthetic fibers, polyamides, polyesters, and celluloses.
An operation known as flushing takes place in the treatment and/or production of pigments.
A solid-state postcondensation takes place in the production and/or treatment of polyester and polyamides; continuous pulping takes place, for example, in the treatment of fibers, e.g., cellulose fibers, with solvents; crystallization from the melt or from solutions takes place in the treatment of salts, fine chemicals, polyols, alkoxides; compounding, mixing (continuous and/or batchwise) take place in the case of polymer mixtures, silicone compositions, sealants, fly ash; coagulation (especially continuous) takes place in the treatment of polymer suspensions.
In a kneader-mixer it is also possible for multifunctional operations to be combined, examples being heating, drying, melting, crystallizing, mixing, degassing, reacting—all continuously or batchwise. Such operations are used to produce and/or treat polymers, elastomers, inorganic products, residues, pharmaceutical products, food products, printing inks.
Other possible operations taking place in kneader-mixers include vacuum sublimation/desublimation, as used to purify chemical precursors, e.g., anthraquinone, metal chlorides, organometallic compounds, etc. Furthermore, pharmaceutical intermediates can be produced.
Continuous carrier-gas desublimation, for example, takes place in the case of organic intermediates, e.g., anthraquinone and fine chemicals.
A kneader-mixer has a continuous gas and product chamber which extends from an intake to a discharge. This differentiates a kneader-mixer essentially from an extruder, where there is no continuous gas chamber between intake and discharge. The continuous gas chamber in the case of the kneader-mixer comes about from the fact that the kneader-mixer is arranged horizontally and is only partly filled with product. Above the product there is a free gas space in which evaporated solvent or the like accumulates.
Single-shaft and double-shaft kneader-mixers differ substantially. A single-shaft kneader-mixer with its shaft disposed horizontally is described in EP 91 405 497.1, for example. Its key characteristic is the presence on the shaft of kneading elements which interact with counter elements that extend radially from an inner housing wall toward the shaft. Usually in this case the kneading elements on the shaft are formed by disk elements with kneading bars mounted on them.
Multishaft mixing and kneading machines are described in CH-A 506 322, in EP 0 517 068 B, in DE 199 40 521 A1, or in DE 101 60 535. In these cases, for example, radial disk elements are located on a horizontally disposed shaft, and axially oriented kneading bars are located on and/or between the disks. Engaging between these disks, from the other horizontally disposed shaft, are mixing and kneading elements of framelike form. These mixing and kneading elements clean the disks and kneading bars of the first shaft. The kneading bars on both shafts in turn clean the inner housing wall. The two shafts may rotate in the same or opposite directions and at the same or different speeds.
The present invention is concerned in particular with the production of a spinning solution. There is no intention, however, for the invention to be confined to this. Cellulose is a common material for producing textile fibers from a spinning solution of this kind. The most simple mode of production is the use of pure cellulose of the kind available in nature. Cotton—which translates from German as “tree wool”—grows as a spinnable fiber in a forb plant, and, after harvesting, sorting, and washing, can be spun directly. The drawback of this method is the comparatively poor availability of cotton plants. These plants also grow only in particular climatic zones, and require intensive watering. Only a small part of the plants is effectively spinnable.
Cellulose is present virtually in all plants, especially in wood. This cellulose has to be purified, with a need in particular to remove hemicellulose and lignin. Unlike cotton, this purified cellulose is not spinnable. It is therefore dissolved in a suitable solvent and extruded through a die. Following emergence of the die, the cellulose is precipitated by washing off the solvent. This produces a fiber of defined diameter, according to the diameter of the spinneret die. This process is called solution spinning. As solvents there are various alternatives. The most widespread is dissolution in xanthate. The product of that process is customarily called viscose, or else rayon in English-speaking countries. Cellulose, physically, is very difficult to dissolve. The viscose process is more of a semi-chemical process, since the cellulose molecules enter into a temporary bond with the xanthate. Another old chemical process is that of acetylation, in which the end product, however, is not cellulose, but rather cellulose acetate.
The xanthate process is highly controversial, resulting in considerable adverse effects on the environment. These adverse effects are only manageable through costly and inconvenient waste-air and wastewater reprocessing techniques. As early as half a century ago, therefore, attempts were made to develop alternatives to this process. The most significant among such processes, and one which is already utilized commercially, is that of dissolution in NMMO as solvent. The product of this process is also called “lyocell”.
With the NMMO process, the purified cellulose is supplied on rolls or in bales and shredded. This cellulose is then swollen with water and various additives. Certain manufacturers do not perform this swelling step. The wet or dry cellulose is then added to a mixture of NMMO and water (NMMO is readily soluble in water). In the case of dry cellulose, the required water content is higher, since in that case the swelling has to take place in the water, cellulose, and NMMO mixture. After mixing, the suspension is gently heated and part of the water is removed under reduced pressure in a vessel with a rotating shaft. Under these circumstances, the cellulose is then soluble in the water/NMMO. The required solids content can be determined fairly reliably by way of the product temperature, since the mixture is close to the thermodynamic equilibrium. It should be borne in mind that this is a ternary mixture, with a boiling point dependent, therefore, not only on the water content but also the ratio of NMMO to cellulose. This ratio is adjusted by the initial mass of the feed, since NMMO and cellulose are both nonvolatile. Customary dissolution equipment used here includes horizontal kneaders or vertical thin-film evaporators with a rotating shaft for the mechanical mixing.
In the conventional process, the ratio of NMMO to cellulose is set such that the resultant solution after the dissolving operation is directly spinnable. It has emerged, however, that there are drawbacks to this process. First of all, the evaporation of water is higher when the water content is higher. Our own measurements have confirmed that the boiling point of the mixture goes up as the water content falls. Accordingly, in the course of the dissolving operation, there is less and less driving gradient for the heat transport of the heating walls. Additionally, there is also a considerable reduction in the heat transfer coefficient as the water content falls. Proposals have therefore been made to admix only a portion of the NMMO to the cellulose, and to admix the rest after the dissolving operation has taken place—in that case, the NMMO which is added at the end can be evaporated separately.
This new process is more economic, since the equipment for the dissolving operation can be smaller and/or higher throughputs can be achieved. One drawback of this procedure, however, is that there is a sharp rise in the viscosity of the solution in the dissolving operation. This has the advantage that mechanical energy input is now also possible, but the drawback that the energy is kneaded in very much more quickly in the viscous range. The user must therefore alternatively 1. Carry out counter-control via the rotary speed, 2. Carry out cooling in the dissolving range, or 3. Shorten the residence time in the mixing zone. In the former case, the output of the dissolver is reduced and mixing is impaired; in case 2., the mixing is impaired, and in case 3. The energy yield is impaired.
Another drawback of this process is that the torque load on the shaft is only local. The dissolution equipment must therefore be given a highly robust construction, including that part of the dissolver where low mechanical loading prevails in continuous operation. If this is not done, there is mechanical damage during the start-up or run-down of the operation or if the operator gives incorrect control orders, if there is fluctuating product quality in the feed, etc. The high local load is also a safety problem. Above particular product temperatures, the NMMO/water/cellulose mixture is highly explosive. Where the torque is locally high, the risk of temperature spikes is greater.
From the prior art it is general knowledge that a mixture is monitored for the alteration of its viscosity. This is known, for example, from U.S. Pat. No. 7,331,703 B1 for ink printers. The monitoring takes place on the basis of the change in the rotary speed of a motor spindle.
Known from US 2006/193197 A1 is a kneader-mixer in which backmixing takes place within the product chamber until the product has reached a predetermined viscosity. This product viscosity is determined overall in the product chamber by measurement of the torque of the drive.
Known from WO 2014/023738 A2, furthermore, is the monitoring of kneading elements distributed over the length of the kneader-mixer. This is applicable in particular to monitoring for breakage or deformation. For this purpose it is also possible, for example, for a torque sensor to be provided that determines those forces acting on the kneading element.
Known from DE 10 2010 014 298 A1 is a method for producing formed articles, more particularly lyocell fibers, in which a base substance for preparing a forming solution is mixed with a solvent and then this solvent is at least partly removed from the mixture and the forming solution is supplied to a means for forming. Accordingly, the forming solution is diluted prior to the forming operation; in other words, the viscosity of the forming solution is influenced.
A problem addressed by the present invention is that of carrying out a process of the abovementioned kind in a substantially more controlled way.